1. Field of Use
The present invention relates to flying prober systems and more particularly to systems for enabling reliable testing of printed circuit assemblies (PCAs).
2. Prior Art
Modern Flying Probers are a class of In-Circuit Test (ICT) equipment which uses a plurality of moving probes in lieu of a standard bed of nails fixture to provide all or most connections between the PCA undergoing test and the test unit. Generally, flying prober tests are a subset of the tests that might have been performed by an ICT unit, because of the limited number of simultaneous connections possible between the PCA and tester. However, in present day test operations, such contact requirements have diminished compared with the recent past.
The principal benefit of flying probers is cost avoidance, in eliminating standard ICT bed of nails fixtures costing tens of thousands of dollars and having a short useful lifespan and little residual value. Another benefit is avoiding the delay associated with constructing such a fixture. Most flying probers have four moving probes, mechanically positionable to any board location by a relatively high speed mechanism. The list of board locations to be used in testing a given PCA type is generally derived from computer aided design (CAD) files provided as part of nearly all modern PCA designs. The same files are similarly used to define nail location points in a bed of nails fixture in non-flying prober testing. Alignment of the PCA on the flying prober is accomplished using electro-optical methods, whereby registration holes of the PCA that would be engaged when mounted on a bed of nails are, instead, found by image recognition methods and their precise locations recorded. Then, instead of mechanically aligning the PCA to the tester, the list of probing locations is recalculated to take into account the actual positions of the registration holes, essentially aligning the tester to the PCA instead of aligning the PCA to the fixture mounted on the tester.
The electro-optical system usually is or is the equivalent of a miniature television camera connected through a digitizer to the computer used to control the tester in its execution of a test program. In addition to its usefulness in PCA to tester alignment, the electro-optical system is used for other purposes related to testing. For example, the probing points may be sighted one by one for the benefit of the test programmer in verifying that the CAD data, upon which the test program is based, indeed matches the PCA for which a test program is being developed. Usually, the television camera used for this purpose is mounted on the carrier that also holds one of the probes. It is mounted in a position that is a predetermined offset from the probe itself. Thus, while the television camera system may not be able to display the probe as it touches the PCA, it can be placed directly over the point where the probe would touch the PCA had not the offset been applied. The camera's optics are aligned perpendicularly to the ideal plane of the PCA, allowing cross hairs or similar positional markings to be added to the image, creating a bombsight effect and allowing confirmation of theoretical probe positioning to a very high degree of precision. This display may also be used to verify the lack of probing obstacles in the vicinity of probing points or that the point is otherwise suitable for probing. One or more additional cameras are sometimes employed to further aide in test programming and/or execution, showing, for example a larger area of the PCA, and at an angle that allows watching some probes as they are extended to touch test points. The electro-optical system employed is sometimes sufficiently complex to allow Automated Optical Inspection (AOI) testing to be performed in conjunction with electrical flying prober tests.
While the nails of a standard ICT bed of nails fixture are mounted perpendicularly to the plane of the PCA while in its test position, flying fixture probing is performed at angles somewhat off perpendicular (Z axis). Angles of between five and sixteen degrees to the Z axis have been noted in some modern flying prober specifications. In some systems, the probe angle may be altered by test program commands. The angles are necessary to allow probing a series of closely spaced points by probes which are, by necessity, attached to relatively large drive mechanisms to allow speedy extension and withdrawal. Those mechanisms are in turn mounted to the carrier driven by an X-Y positioning mechanism. Two types of X-Y mechanisms used are linear motor and lead screw. Furthermore, the probes may be at angles to the X or Y axis as well as the Z axis. A single X-Y table of probe points suffices, regardless of the number of probes, variety of angles, or thickness of the particular PCA type being tested, by applying appropriate offsets as compensation for these effects in determining the precise point at which the PCA will be contacted.
However, the compensation discussed above is based upon the assumption that the probing points of the PCA exist in a perfect plane, or at predetermined differences from a perfect plane. Warpage of the PCA is both non-planar and unpredictable. Hence, planarity variations result in probing variations. In some cases, the intended test probing target may be probed slightly askew from the intended point of contact, usually the center. In other cases, the probe may miss the target altogether. For example, consider the case of a probe which is fifteen degrees from perpendicular attempting to probe a target point which is 35 mils in diameter. The required probing accuracy would be +/−17 mils, assuming the probe will not slide once one physical contact is made (not a safe assumption). At an angle of fifteen degrees, a 17 mil error occurs when the height of the intended target is approximately 1/16″ (0.017″/0.268, the tangent of 15 degrees). The actual safe region for contacting a 35 mil target with a 15 degree probe is a matter of opinion. If, however, half the error were considered safe (about 8 mils), the height would have to be predictable to within approximately 1/32″. Maintaining a planarity tolerance of +/− 1/16″ is generally not possible in a manufacturing environment with PCAs measuring 16″ by 16″ or more. Even with larger test targets, the planarity requirements are often impractical to maintain.
In the art of PCA testing using flying probers, planarity variations that cause a misprobe are a known problem, but there is a paucity of detailed data as to its significance. One result of a misprobe is a false error (e.g., when testing for resistance and getting an open indication) or a missed error (e.g., when a short was present but not detected). But, there are so many possible reasons for such errors, such a large quantity of such errors and so little engineering time to devote to making exact determination of error causes (real vs. false), that the effectiveness of available attempted solutions to the planarity problem has never been fully tested. Such attempted solutions are, e.g., standoff posts used to support a concave PCA from the underside during probing (but which may have no effect on convex PCAs) and standoff posts which attempt to apply either an upward or downward force, as needed, by means of vacuum applied over so small an area as to effect only relatively flexible PCAs. In all such cases, the required planarity cannot be guaranteed in a production environment.
Misprobes may also occur when the probing target is large enough that it will not be missed because of height differences. In such cases, the force applied by the probe may be inadequate, causing a lack of contact between the probe and PCA or too great, causing marking of the contact area. The marking may be in the form of a pit or a scored line, the latter resulting when the probe is pushed by excessive force. The marking occurs because probe contact depends upon spring force. In normal operation, contact force is achieved by attempting to drive the probe perhaps 50 to 100 mils further than would be required for the tip to make contact with the PCA probing point. At contact, the probe will stop moving and its internal spring will compress to take up the distance, providing contact pressure. In a case where the PCA contact point was significantly closer to the probing mechanism, early contact would be made and the probe might be driven 100 mils before the 100 mils previously referenced, for a total of 200 mils. In some cases, the spring might even fully compress, causing the probe to be driven against the PCA with the maximum force the probe extension motor can produce. In certain applications, the significant marking of PCA targets is not tolerated and the PCA has to be reworked or scrapped.
Accordingly, it is a primary objective of the present invention to provide a method and apparatus for overcoming variations in PCA board planarity of PCAs mounted for testing in a flying prober system.
It is a further objective of the present invention to utilize extensively hardware typically found on existing flying prober systems, making the retrofitting of such systems to incorporate the present invention practical.